u, sajolida: is this a blocker for the release? If so, please bump priority to Elevated. The impact is unclear to me as we don't include gnome-software by default, but the fix seems to trivial that it probably takes less time to just fix it than to discuss it any further.

It still fails to display other available apps in the "All" tab until I run apt update to download the DEP-11 metadata (the config file that enables this wasn't there before the appstream package is pulled in by installing GNOME Software itself), but that's another problem, that should get its own ticket if/once we decide to ship GNOME Software in Tails.

When starting software-properties-gtk I still get:

[...]

This comes from https://sources.debian.net/src/python-apt/1.4.0%7Ebeta3/aptsources/distro.py/?hl=92#L92. The templates live in /usr/share/python-apt/templates/ and contain a list of mirrors that aptsources/sourceslist.py will use when parsing the existing APT sources. So as the error message says, there's no Tails template in there. We could add some, based on the Debian ones (and perhaps we'll need to add entries for the non-Debian APT sources we use?) but the Debian hidden services we're using are not listed in the Debian template so that's not enough: copying Debian.{info,mirrors} to Tails.{info,mirrors} fixes the startup of software-properties-gtk, and then the first tab is labelled "Tails Software" (wow!) with every checkbox unchecked, and all our APT sources are listed in the "Other Software" tab.

IMO we have two options: either display all the APT sources we enable by default in "Tails Software" (so that user-added ones are clearly singled out, and "Tails" appears as a merge of Debian and non-Debian sources, which is really is), or pretend we're Debian (and then set our name to Debian in /etc/os-release, display all the Debian sources in "Debian Software", and all our others sources in "Other Software"). I prefer the first option personally, but I want sajolida's input on this before implementing anything, since he's the one who wants to design/document stuff in this interface.